The physically immature horse

Horse Health: Be mindful of the young horse’s developmental stages to protect its future health

Excessive fluid in the hock joints of a young horse such as this three-year-old filly, is an indication that the joint is experiencing developmental distress and is at a significantly higher risk to develop arthritis as the horse ages. Photo:
Carol Shwetz

Given the increasingly common occurrence of arthritis in today’s horses, it is worthwhile to consider the choices that are made in a young horse’s life that are large contributors to the outcome of that horse’s soundness over its lifetime.

Oftentimes arthritis is viewed as a condition of the aging horse, mostly because the outward clinical signs of pain and stiffness typically tend to become more evident later in life. However, once the discomfort appears, the degenerative processes of arthritis are well underway, and typically have been for quite some time. Pain management and therapies attempt to slow the progress and momentum of the degenerative processes, but unfortunately they are not able to cure the disease.

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It is important to understand that other than withstanding an injury, the majority of the arthritic conditions in horses are rooted in the way a horse is fed, managed, trained or used for many years beforehand.

The care a horse receives in every phase of its life will ultimately affect if, when, and to what extent arthritis may develop. However, it is the care that the young horse receives during the phase of bone growth and development which becomes of utmost importance to its soundness as the horse ages. This is because the developing bones of a young horse pass through an immature and “soft” cartilaginous stage prior to becoming the mature “hard” bones of an adult horse.

Nature takes her time to properly develop the skeleton of big mammals, and horses are no exception. Many horse owners are surprised to discover that the horse is skeletally and physically immature for the first 5-1/2 to six years of its life — maybe even older, such as seven, in some of the larger breeds.

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During this time the young horse’s skeleton is “soft” and highly susceptible to improper development and damage. The term “soft” is used to describe the immature cartilaginous sites located throughout the horse’s skeleton prior to its transition through mineralization of calcium salts into hard and mature bone. These cartilaginous sites, referred to as “growth plates” are numerous throughout the equine skeleton. These sites serve as ossification centres for bone mineralization and are highly active metabolically — like tiny biological construction zones.

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In the case of some bones, like the pelvis or vertebrae, there are multiple “corners” and growth plates. While these cartilaginous sites are transitioning into mature, strong and stable bone they are highly susceptible to repercussions from nutritional and environmental insults which interrupt the conversion stage of cartilage. This “soft” tissue is particularly vulnerable to structural damage if improper nutrition occurs during its formation and transition. This immature cartilaginous bone is also highly susceptible to damages suffered from physical overburdening, traumatic injury and mechanical stressors, especially forces of torque and sheer.

Faults, flaws and damage incurred during the transition from cartilage to bone is unforgiving and often permanent because the horse has limited capacity to recapture this formative process of cartilage development and correct the faulty framework. The mineralization process continues despite the damaged and unstable cartilage scaffolding and the horse is fated to develop various degrees of arthritic ails at some point during its lifetime. Just like rebar placement is vital to the integrity and lifespan of cement foundations, so is the mineralization of a “correct” and “proper” cartilaginous framework. This “soft” developing bone is the forerunner to a steadfast bone foundation so crucial to the level of soundness in a mature horse. The link between interruption of this conversion process in the young horse and the soundness of a horse as it ages are identifiable, traceable and repeatable in many cases of arthritis.

So, firstly the healthy musculoskeletal system of a horse begins with an understanding of the broodmare’s nutritional needs, for it is in the womb where the initial “scaffolding” for the skeleton of the young foal is forged and this scaffolding needs the proper physical resources.

Once birthed, there exists a schedule of growth plate closure in the immature horse that extends often into the sixth year. In general, the schedule for growth plate fusion in the horse begins from the ground up with developed ossification occurring in the lower limb bones like the coffin, pasterns, and cannon bones firstly. The growth plates above the knee in a three-year-old remain largely unfused i.e. most importantly those of the hock and those of the horse’s spine with those of the lower cervical vertebrae being the final centres of ossification to close.

If the ossification process is disturbed or damaged and thus unable to complete properly, “pockets” of unstable and fragile bone occur — not unlike biological Styrofoam. If these “pockets” underlie joint surfaces or are positioned to act as anchor points for the crucial supportive tissues of joint such as collateral ligaments the instability becomes host to a number of developmental bone and joint pathologies with various labels, one of which is arthritis.

Understanding this “window of vulnerability” in bone development is important for insightful decision-making in the young horse’s life since these decisions influence the soundness of the horse’s bones and joints as they age.

Young horse bodies need proper nutritional care, reasonable training schedules and, very importantly, TIME to be young horses in order to develop healthy musculoskeletal structures for a lifetime of soundness.